This invention concerns sensitive key devices with capacitive effect used particularly in numeric keypads, for example for a kitchen hob.
It is known that this type of device is capable of detecting the presence of an element (a user's finger or an electrically conducting object such as a metal saucepan) through an electrically insulating body like a glass or vitroceramic plate.
More precisely, placing a finger or an object on a plate in an area opposite a sensitive key will cause a change in capacitance. The key is electrically connected to an electronic circuit that detects this change in capacitance and transforms it into an active or inactive state.
Many embodiments of keys that operate based on the above principle have already been suggested. These solutions are particularly concerned with specific embodiments in which the key and components forming the electronic key activation detection circuit are placed on an electrically insulating support located behind the plate.
In a first known embodiment, this electrically insulating support is composed of two printed circuit boards facing each other and maintained at a given spacing, one of the two boards mainly supporting the keys obtained by engraving, and the other supporting mainly the electronic components. In this embodiment, manufacturing costs are considerably increased by difficulties in forming the mechanical and electrical connections when the two plates are assembled. Furthermore, even a minor defect in the plane of the board supporting the keys which is applied in contact with the internal surface of the transparent plate will cause air gaps between some keys and the transparent plate. This results in unsatisfactory operation due to differences in the sensitivity between the keys making up the keypad, or sometimes complete insensitivity of some keys.
In order to overcome these disadvantages, a second known embodiment proposes an electrically insulating support composed of a single printed circuit board containing both keys and electronic components. Each key contains an electrically conducting head mechanically connected to the printed circuit board by a first device capable of moving transversally with respect to the printed circuit board under the action of a second device comprising an electrically conducting spring. This spring forms the electrical link between the top of the key and the electronic circuit, and transverses displacement of the first device so that the head is in contact with the transparent plate. This type of embodiment is described in document FR-A-2.704.332. It overcomes the air gap problems mentioned above, but the number of distinct parts necessary to make the key remains high. These parts must be manufactured independently of each other which increases the global manufacturing cost, and assembly of these parts remains complicated. Furthermore, there is a high risk of a defect in the electrical contacts.